The hardest 10 I've done (trying not to take the weather into account)
Hardest to easiest...
1. Midlander Super Grimpeur 300
2. Bryan Champan 600
3. Elenith 300
4. Dean 300
5. Anfractuous 200
6. Severn Across 400
7. LEL 1400
The rest I've done are relatively tame.
Various DIYs may have qualified (Dun Run and back home as a 400, Putney-Cambridge-Putney-RoyalTunbridgeWells-Bethersden-FlattestPossible300 as a 600) but that's because DIYs are more of a mental struggle (despite having good company all the way round) than a physical struggle.
Some rides felt tough because of the weather. The Dean in 2008 (wind, rain, blizzards) was much much harder than in 2009 (relatively calm with just a few showers) for example. Both of my Eleniths have had roughly similar weather (early downpour vs late sleet/snow) but the first was on gears and the second on fixed.